1. Field of the Invention
An embodiment of the disclosed invention relates to a method for charging a lithium ion secondary battery and a battery charger.
2. Description of the Related Art
In recent years, with the advance of environmental technology, development of power generation devices (e.g., solar power generation devices) which pose less burden on the environment than conventional power generation methods has been actively conducted. Concurrently with the development of power generation technology, development of power storage devices such as lithium ion secondary batteries, lithium ion capacitors, and air cells has also been underway.
In particular, demand for a lithium ion secondary battery which is used as a storage battery for a next-generation clean energy vehicle such as a hybrid electric vehicle (HEV), an electronic vehicle (EV), or a plug-in hybrid electric vehicle (PHEV), or a small consumer product such as a portable information terminal (e.g., a mobile phone, a smartphone, or a notebook personal computer), a portable music player, or a digital camera has rapidly expanded, and the lithium ion secondary battery becomes an indispensable rechargeable energy supply source.
A lithium ion secondary battery basically has a structure in which an electrolyte is provided between a positive electrode and a negative electrode. Typically, a positive electrode and a negative electrode each include a current collector and an active material provided over the current collector. A material which can occlude and release lithium ions is used for the active materials of the lithium ion secondary battery (see Patent Document 1).
In the case where graphite is used for a negative electrode active material layer and ethylene carbonate is used for a solvent of an electrolyte of a lithium ion secondary battery, the solvent is reduced and decomposed and a passivating film (also referred to as a solid electrolyte interface (SEI)) is formed on a surface of the negative electrode active material layer. The passivating film prevents the electrolyte from being further decomposed, and lithium ions can be inserted. In addition, stability of the passivating film influences on safety of the whole lithium ion secondary battery because the passivating film protects the negative electrode active material layer (see Patent Document 2 and Non-Patent Document 1).
As a method for charging a lithium ion secondary battery, constant current/constant voltage charge (CCCV charge) in which charge is performed with constant current and then with constant voltage is developed, which is used as a general method for charging a secondary battery (see Patent Document 3).